How to Make It
Preheat the oven to 300°. In a small baking dish, drizzle the tomato quarters with the olive oil and scatter the garlic on top. Add the thyme sprig to the tomatoes and season with salt and pepper. Toss well and arrange the tomatoes skin side up. Bake for about 1 hour, or until very soft. Peel the tomato quarters. Leave the tomatoes in the dish and discard the thyme sprig. Leave the oven on.
Meanwhile, in a small saucepan, combine the 3/4 cup of vegetable oil with a little more than half of the sliced lemongrass and bring to a boil. Simmer over low heat for 10 minutes. Remove from the heat and let steep for 30 minutes. Strain the oil.
In a medium saucepan, melt 2 tablespoons of the butter. Add the shallot and cook over moderate heat until softened, about 3 minutes. Add the remaining lemongrass and the lemon zest and cook over low heat for 10 minutes, stirring occasionally. Add the wine and simmer until reduced by half, about 3 minutes. Add the cream and simmer over low heat until reduced to 1/2 cup, about 15 minutes. Strain the sauce into a small saucepan.
In a medium saucepan of boiling water, cook the scallions for 1 minute. Using tongs, transfer them to a plate. Cut the scallions into 2-inch lengths. Add the leek julienne to the water and blanch for 1 minute. Drain and pat dry with paper towels.
In a medium skillet, heat 1/4 inch of vegetable oil until shimmering. Add the leek and fry over moderate heat until crisp but not browned, about 3 minutes. Using a slotted spoon, transfer the leek to paper towels and drain. Season with salt.
Put the scallions in the dish with the tomato and warm in the oven. Meanwhile, in a large skillet, warm 1/4 cup of the lemongrass oil until shimmering. Season the bass fillets with salt and pepper, add them to the skillet and cook over moderately high heat until lightly browned and just opaque, about 3 minutes per side. Transfer the fillets to plates and arrange the tomato quarters and scallions on top.
Bring the lemongrass cream sauce to a boil. Remove from the heat and stir in the lemon juice, then whisk in the remaining 1 tablespoon of butter until smooth. Season with salt. If desired, froth the sauce using an immersion blender. Spoon the sauce around the fish and drizzle some of the lemongrass oil around the plate. Top with the fried leek julienne and serve.
The remaining lemongrass oil can be refrigerated for up to one week. It can be used to sauté seafood, chicken or pork. Or it can be used to make a vinaigrette or as a garnish for finished dishes.